U.S. Pat. No. 4,670,641 entitled: Machine robot, particularly for welding; describes a robot arm for use with an industrial robot. The robot arm has a multiplicity of degrees of freedom. The various parts of the robot arm are rotatable about a plurality of axes. A servo-system for one of the degrees of freedom is capable of disengaging a part of its feedback network, whereby the adjustment passes from Proportional Integral (PI type) control to Proportional (P-type) control. This is done because the accuracy and precision of the robot arm control can surpass the precision of part of the workpiece being welded or otherwise processed. The fine adjustment of the servo controlled movement of the robot arm, and thereby the tool operated, is removed and movement is temporarily regulated in two axes by means of a calculating circuit. The calculating circuit calculates, in this case, two component signals, each being a weighted sum of the difference between the real (measured) values and the or each reference or set point values two of the axis systems.
U.S. Pat. No. 6,477,445 entitled: Method and apparatus for controlling an Industrial Robot; describes a control and regulation system which, amongst other things, has the function of a servo that is also very sensitive to forces imposed externally on the robot arm, tool, gripper and so on. This enables an operator to teach a robot a task by literally taking hold of the robot arm/tool and moving it to specific points as desired. During this time the robot arm and/or tool appear to the operator to be almost weightless. This functionality of a robot is normally only provided when a robot is under what is usually described as a “compliance control” mode of the robot control system. Movements in response to externally imposed forces such as the above are also described as compliant movements. The weight of the robot arm is borne by the various robot arm drive motors under the fine control of the servo control program during such a configuration or teaching stage.
The robot servo control system described above normally performs well in service. However with some robots having gearboxes that have high gear ratios, and for some larger robots with heavier parts, and also under conditions of varying temperature, there is a tendency of increased friction and variation in friction under operation that makes accurate movement in response to externally applied forces difficult. Increased friction is also due in part to the use of pre-stressed bearings and pre-stressed gears which improve structural stiffness of the various components but cause increased loads on the bearings, and thus increased friction. Another problem is that reaction forces can arise sometimes in a second axis in response to movements of a large amplitude in a first axis, which further complicates accurate control of the forces in one or more axes.